Performance of floating and sunken media biological aerated filters under unsteady state conditions

The biological aerated filter (BAF) combines both biological treatment and solids removal in one submerged, aerated three phase reactor. Over the last 20 yr biological aerated filters have been developed not only as competitors for other secondary treatment systems such as trickling filters and activated sludge plants but also as tertiary treatment systems. The aim of this work was to compare the performance of two such reactors, one containing a floating media and the other containing a sunken media under identical conditions during start-up and with increasing hydraulic flowrates. Two methods of start-up were used. The first method used activated sludge as seed which was recycled through the reactor, the second method involved simply passing the process liquid (settled domestic sewage) through the reactor at the initial flowrate. Following a period of steady-state the flowrates were increased incrementally and the performance of the reactors analysed during and after each increase. The results for the start-up indicated that the overall time to reach steady-state using both methods was approximately the same but due to the initial time required for the recycling of the activated sludge, this method took slightly longer. From an initial start-up loading of 0.486 kg m−3 d−1 (SS) and 0.568 kg m−3 d−1 (sCOD) suspended solids and soluble chemical oxygen demand removal efficiencies dropped to below 50% when loadings of 1.397 kg m−3 d−1 (SS) and 1.403 kg m−3 d−1 (sCOD) were reached. The floating media performed better at the higher flowrates under shock loading conditions than the sunken media.